Generally a female will sit on the toilet (lavatory) seat, whilst the other female will stand with her feet in a sports bag, whilst the seated female, drinks from the furry cup and loses some fingers. To any unsuspecting passer by, who just happens to peer under the cubilce door, only one pair of feet/legs will be seen, so the encounter will be undetected.

Girl 1 Hey did you know Gloria and Maggie were Female Cottaging in the staff toilet?
Girl 2 No. I've only ever seen them in there one at a time
Girl 1 Why do you think Gloria always carries that empty sports bag